van Elmpt Wouter J C, Nijsten Sebastiaan M J J G, Schiffeleers Robert F H, Dekker André L A J, Mijnheer Ben J, Lambin Philippe, Minken André W H
Department of Radiation Oncology (MAASTRO), GROW U.H. Maastricht, Maastricht, The Netherlands.
Med Phys. 2006 Jul;33(7):2426-34. doi: 10.1118/1.2207315.
The verification of intensity-modulated radiation therapy (IMRT) is necessary for adequate quality control of the treatment. Pretreatment verification may trace the possible differences between the planned dose and the actual dose delivered to the patient. To estimate the impact of differences between planned and delivered photon beams, a three-dimensional (3-D) dose verification method has been developed that reconstructs the dose inside a phantom. The pretreatment procedure is based on portal dose images measured with an electronic portal imaging device (EPID) of the separate beams, without the phantom in the beam and a 3-D dose calculation engine based on the Monte Carlo calculation. Measured gray scale portal images are converted into portal dose images. From these images the lateral scattered dose in the EPID is subtracted and the image is converted into energy fluence. Subsequently, a phase-space distribution is sampled from the energy fluence and a 3-D dose calculation in a phantom is started based on a Monte Carlo dose engine. The reconstruction model is compared to film and ionization chamber measurements for various field sizes. The reconstruction algorithm is also tested for an IMRT plan using 10 MV photons delivered to a phantom and measured using films at several depths in the phantom. Depth dose curves for both 6 and 10 MV photons are reconstructed with a maximum error generally smaller than 1% at depths larger than the buildup region, and smaller than 2% for the off-axis profiles, excluding the penumbra region. The absolute dose values are reconstructed to within 1.5% for square field sizes ranging from 5 to 20 cm width. For the IMRT plan, the dose was reconstructed and compared to the dose distribution with film using the gamma evaluation, with a 3% and 3 mm criterion. 99% of the pixels inside the irradiated field had a gamma value smaller than one. The absolute dose at the isocenter agreed to within 1% with the dose measured with an ionization chamber. It can be concluded that our new dose reconstruction algorithm is able to reconstruct the 3-D dose distribution in phantoms with a high accuracy. This result is obtained by combining portal dose images measured prior to treatment with an accurate dose calculation engine.
调强放射治疗（IMRT）的验证对于充分的治疗质量控制是必要的。治疗前验证可以追踪计划剂量与实际输送给患者的剂量之间可能存在的差异。为了评估计划光子束与实际输送光子束之间差异的影响，已经开发了一种三维（3-D）剂量验证方法，该方法可在体模内重建剂量。治疗前程序基于使用电子射野成像装置（EPID）测量的各单独射野的射野剂量图像，射野中没有体模，以及基于蒙特卡罗计算的三维剂量计算引擎。测量的灰度射野图像被转换为射野剂量图像。从这些图像中减去EPID中的侧向散射剂量，并将图像转换为能量注量。随后，从能量注量中采样相空间分布，并基于蒙特卡罗剂量引擎在体模中启动三维剂量计算。将重建模型与不同射野尺寸的胶片和电离室测量结果进行比较。还对使用10 MV光子输送到体模并在体模中几个深度处用胶片测量的IMRT计划测试了重建算法。对于6 MV和10 MV光子，深度剂量曲线均被重建，在大于建成区的深度处，最大误差通常小于1%，对于离轴剖面，不包括半影区，误差小于2%。对于宽度范围为5至20 cm的方形射野尺寸，绝对剂量值重建误差在1.5%以内。对于IMRT计划，使用伽马评估将剂量重建并与胶片剂量分布进行比较，标准为3%和3 mm。照射野内99%的像素伽马值小于1。等中心处的绝对剂量与电离室测量的剂量在1%以内相符。可以得出结论，我们新的剂量重建算法能够高精度地重建体模中的三维剂量分布。这个结果是通过将治疗前测量的射野剂量图像与精确的剂量计算引擎相结合而获得的。
